Replace Sphinx with Mkdocs

[AnesBenmerzoug]

Description

This PR closes #261

Branches merged into this one close #245, close #308, close #179, close #405

What's left to be done:

• Finish updating all docstrings (switch to Google docstring style, replace all rst directives).
• Redo influences package's docstrings.
• Add notebooks to documentation.
• Update contributing docs with explanations and instructions.
• Add support for multiple versions.
• Remove sphinx left-over files (conf.py, update_docs.py)
• Update PR template to suggest using the "hide" and/or "hide-input" cell tags instead of "nbsphinx":"hidden" for notebooks.
• Test building docs on CI
• Test building multiple version of docs on CI

Changes

• Add mkdocs and its plugins as dependencies in a new requirements-docs.txt file
• Replace rst files with md files.
• Convert home, installation, getting started, data valuation and influence function docs.
• Temporarily copy notebooks to docs dir to add them to documentation.
• Temporarily copy changelog to docs dir to add it to documentation.
• Customize default theme.

Checklist

• Wrote Unit tests (if necessary)
• Updated Documentation (if necessary)
• Updated Changelog
• If notebooks were added/changed, added boilerplate cells are tagged with "nbsphinx":"hidden"